JAVA QUESTION: Why I still get an error "Editor does not contain a main type"?

//Exercise_13_HarvestTask.java

import becker.robots.*;

public class Exercise_13_HarvestTask extends Object

{

public static void main(String[] args)

{

City stLouis = new City("Exercise_13_Harvest.txt");

Exercise_13_Harvester mark = new Exercise_13_Harvester(stLouis, 1, 0, Direction.EAST, 0);

mark.move();

mark.harvestField();

mark.move();

for (int tries = 0; tries < 40; ++tries)

{

mark.putThing();

}

}

}